1. Field of the Invention
The present invention relates to secondary air supplying apparatuses for supplying secondary air to upstream of an emission purifying device of an internal combustion engine.
2. Related Background Art
An example of the known emission control systems of internal combustion engine is an apparatus with a three way catalyst in the exhaust system to reduce CO, HC, and NOx components in exhaust gas and thereby clean up exhaust emissions. Furthermore, there is a known technique of connecting a secondary air supply path equipped with a switching valve, to an exhaust manifold, and pressure-feeding air (secondary air) into the exhaust manifold by means of an air pump to increase the oxygen content in exhaust, thereby effecting the cleanup based on promotion of oxidation of HC and CO in exhaust (Japanese Patent Application Laid-Open No. 2000-240434: [0018]–[0030], FIGS. 3, 7–9).
The technology described in the above application is one for supplying the secondary air neither too much nor too little. The length of a duration during which the engine is kept at a stop is determined based on a difference between a coolant temperature at a stop of the engine and a coolant temperature at a start with the technology. When the duration of the engine stop is determined to be short, an actuation period of a secondary air supply system is adjusted according to a temperature difference between the coolant temperature at the start and an intake air temperature. Then the foregoing operation can suppress overheating of the catalyst and implement appropriate activation by shortening the actuation period of the secondary air supply system, when the duration of the engine stop is short and the temperature of the catalyst is high.
However, there are cases where an increase speed of the catalyst temperature is faster than an increase speed of the coolant temperature and where the catalyst has been already activated even at relatively low coolant temperatures. In such cases, where the actuation period of the secondary air supply system is too long, there are possibilities of causing overheating of the catalyst and damage due to continuous operation of the air pump.